The present invention relates to x-ray tube technology. More specifically, the present invention relates to a method and apparatus for directing cooling fluid supplied from a reservoir to (i) a bearing cooling apparatus and (ii) an x-ray tube housing chamber to reduce the heating effects on x-ray tube bearings caused by heat dissipated from the anode during operation.
Conventional diagnostic use of x-radiation includes the forms of (i) radiography, in which a still shadow image of the patient is produced on x-ray film, (ii) fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and (iii) computed tomography (CT) in which complete patient images are digitally constructed from x-rays produced by a high powered x-ray tube rotated about a patient""s body.
Typically, an x-ray tube includes an evacuated envelope made of metal or glass which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The fluid is circulated through the housing and a heat exchanger external to the housing for removing heat from the cooling fluid. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination.
In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly. The cathode assembly includes a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode assembly and the anode assembly. This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the envelope. A cathode focusing cup containing the cathode filament focuses the electrons onto a small area or focal spot on a target of the anode assembly. The electron beam impinges the target with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through the x-ray transmissive windows of the envelope and x-ray tube housing to a beam limiting device, or collimator, attached to the x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination thereby allowing images to be constructed.
In order to distribute the thermal loading created during the production of x-rays a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path about a peripheral edge of the target. Each portion along the circular path becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam. In many high powered x-ray tube applications such as CT, the generation of x-rays often causes the anode assembly to be heated to a temperature range of 1200-1400xc2x0 C., for example.
In order to provide for rotation, the anode assembly is typically mounted to a rotor which is rotated by an induction motor. The rotor in turn is rotatably supported by a bearing assembly. The bearing assembly provides for a smooth rotation of the rotor and anode assembly about its axis. The bearing assembly typically includes at least two sets of ball bearings disposed in a bearing housing. The ball bearings often consist of a ring of metal balls which are lubricated by application of lead or silver to an outer surface of each ball thereby providing support to the rotor with minimal frictional resistance.
During operation of the x-ray tube, the anode assembly is passively cooled by use of oil or other cooling fluid flowing within the housing which serves to absorb heat radiated by the anode assembly through the envelope. However, a portion of the heat radiating from the anode assembly is also absorbed by the rotor and bearing assembly. For example, heat radiated from the anode assembly has been found to subject the bearing assembly to temperatures of approximately 400xc2x0 C. in many high powered applications. Unfortunately, such heat transfer to the bearings may deleteriously effect the bearing performance. For instance, prolonged or excessive heating to the lubricant applied to each ball of a bearing can reduce the effectiveness of such lubricant. Further, prolonged and/or excessive heating may also deleteriously effect the life of the bearings and thus the life of the x-ray tube.
One known method to reduce the amount of heat passed from the anode assembly to the bearing assembly is to mechanically secure a heat shield to the rotor. The heat shield serves to protect the bearing assembly from a portion of the heat radiated from the anode assembly in the direction of the bearing assembly. Unfortunately, heat shields are not able to completely protect the bearing assembly from heat transfer from the anode assembly and a portion of the heat radiated will be absorbed by the bearing assembly. Additionally, although the heat shield is useful in preventing some heat transfer to the bearing assembly, the heat shield does not play a role in cooling the bearing assembly by removing heat already absorbed therein. Further, given that the bearing assembly is enclosed by the rotor, the bearing assembly is not able to easily radiate heat to the cooling fluid contained in the housing as done by the anode assembly. In fact, some rotor and bearing assembly configurations operate as a heat sink. For these reasons, a substantial amount of heat is typically transferred into the bearing assembly and the heat is not readily dissipated.
Another method to reduce heating of bearings is to pass cooling fluid through an internal conduit in the bearing assembly. For example, as described in U.S. Pat. No. 6,011,829, cooling fluid is supplied through two separate input tubes from a heat exchanger into the x-ray tube housing. A first supply tube provides cooling fluid through a first opening in the housing to be directed to a cooling fluid shaft along an inner surface of the bearing housing. A separate second supply tube provides cooling fluid through a second opening in the housing directly into the chamber surrounding the x-ray tube. A fluid flow regulator consisting of conventional valve controls is located outside the tube housing in the heat exchanger. The regulator valves control the flow rate of cooling fluid through each of the respective inlet tubes and openings in the housing wall. A third cooling fluid return port circulates the cooling fluid back to the heat exchanger. However, it is desirable to reduce the number of supply tubes, openings and fluid connections in the housing. In addition it is desirable to simplify the fluid flow regulator.
Therefore, what is needed is an apparatus for effectively and simply directing the appropriate volume of cooling fluid into each of (i) the chamber within the housing that surrounds the x-ray tube and (ii) the cooling fluid shaft along the inner surface of the bearing housing for the x-ray tube located within the housing.
In accordance with the present invention, an x-ray apparatus is provided. The x-ray apparatus includes a housing defining a chamber. The x-ray tube housing has a fluid input port. The x-ray tube includes a cathode assembly having a filament which emits electrons when heated, an anode assembly defining a target for intercepting the electrons such that collision between the electrons and the anode assembly generate x-rays from an anode focal spot and a bearing assembly rotatably supporting the anode assembly. The bearing assembly includes a fluid channel for providing a flow of fluid across a surface of the bearing assembly. An envelope encloses the anode assembly, the cathode assembly and bearing assembly in a vacuum. The invention includes a fluid director received in the fluid input port, the fluid director has a fluid input aperture, a first fluid output aperture operatively connected to provide fluid into a first fluid path and a second fluid output aperture to provide fluid into a second fluid path. The fluid input aperture is in fluid communication with both of the first and second fluid output apertures.
In a more limited aspect of the invention, the first fluid path includes the fluid channel.
In a further limited aspect of the invention, the fluid channel is internal to the bearing assembly.
In another limited aspect of the invention, a portion of each of the first and second fluid paths is common to both fluid paths.
Yet another limited aspect of the invention includes establishing the size of the first fluid output aperture and second fluid output aperture in a predetermined ratio to provide a desired portion of the supplied flow of fluid through at least one of the first and second fluid output aperture.
In a more limited aspect of this invention, the fluid flow from the first and second fluid output apertures is equal.
In another limited aspect of the invention, the fluid director includes a wall portion that defines a cavity in fluid communication with the input aperture. The wall portion includes a side wall portion and an end wall.
In a more limited aspect of the invention, the first fluid output aperture is in the end wall and the second fluid output aperture is in the side wall.
In another more limited aspect of the invention, the area of the second fluid output aperture is divided into a plurality of apertures that provide fluid flow into the second fluid path.
In yet another more limited aspect of the invention, the fluid director includes a tubular member connecting the first fluid path with the first fluid output aperture.
In accordance with the present invention, a method for cooling a bearing assembly in an x-ray tube includes the step of supplying fluid flow through a fluid input aperture into a cavity of a fluid director. The fluid director is located in a housing of an x-ray tube assembly and the housing of the x-ray tube assembly defines a chamber. The method further includes the step of directing a predetermined portion of the fluid flow supplied into the cavity out a first output aperture into a first fluid path. The first fluid path includes a cooling channel along a surface of the bearing assembly. Another step in the method of the present invention is directing the remaining fluid flow into the chamber through a second fluid path that does not include the cooling channel in the bearing assembly of the x-ray tube.
One advantage of the present invention is that cooling fluid is directed into different fluid flow paths using a fluid flux director. The present invention provides fluid at the predetermined portion of supply fluid into each path.
Another advantage of the present invention is that supply of the plurality of fluid paths with their specific fluid requirements may be accomplished without additional pumps being installed in the system. This is particularly advantageous in Computed Tomography systems in which the X-Ray Tube housing assembly, including the fluid systems, is rotated around a gantry.
Yet another advantage of the present invention is that there is only a single input port for the cooling fluid through the housing. Two different fluid flow requirements are served with a single fluid input port.
Another advantage of the present invention is that it permits the retrofit installation of x-ray tube inserts having fluid cooled bearing assemblies into existing systems which are not so equipped. The installation of the fluid flux director into the presently existing input port of an existing x-ray tube housing facilitates the simultaneous retrofit installation of an x-ray tube having a fluid cooled bearing assembly.
And yet another advantage of the present invention is the structure of the fluid flux director having a predetermined size for the fluid output apertures to achieve the desired portion of fluid flow into each fluid path.
To accomplish the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.